TECHNOLOGY AREA(S): Materials
OBJECTIVE: Develop an advanced additive manufacturing technology for constructing functionally graded radomes for use in missiles and other hypersonic vehicles.
DESCRIPTION: Radome is a cover or enclosure that protects radar antennas from environmental influences, and made from an electromagnetically transparent material. The radome can have a huge influence on sensitivity, radiated antenna pattern and immunity to vibrations. Minimization of microwave reflection at the surface of the cover should be a key aspect in designing a radome, and therefore, materials with low dielectric constant (< 5) and low loss tangent (< 5~10%) are needed without compromising thermal and mechanical characteristics required for the target environment. For hypersonic vehicles at Mach 5~10 (1~2 miles/sec), the temperature of the aircraft can reach anywhere from 1500C to 2000C. Thus, the radome materials must also satisfy certain unique thermal and mechanical requirements relevant to the harsh operating environment. Among them, high melting temperature >3200F (or >1760C), high flexural strength >50-100 MPa and high Young’s modulus > 50-100 GPa are critical parameters. In addition, high thermal conductivity, low water absorption, low density, high particle, rain and thermal impact resistance, and high mechanical strength, hardness, and flexibility are also important characteristics depending on the application. Different radome wall structures have been used, including half-wave wall, thin wall, A-sandwich, B-sandwich, C-sandwich, multilayer, and graded radome wall structures. Half-wave wall or thin wall radomes are individual layer radome materials suitable for narrow band applications. Layered and graded radome wall structures are used for broadband radome applications. Conventional sandwich or layered radome wall structures are typically fabricated by epoxy bonding which has a limited range of operation temperature and, therefore, they suffer from delamination at high operation temperatures due to mismatched thermal expansion coefficients. Functionally graded radome wall structures enable the combined properties for hypersonic radomes, and are under intensive research. Additive manufacturing processes are based on layer-by-layer manufacturing, which constitute an excellent fit for fabricating the functionally graded radomes for hypersonic vehicles. At the same time, the additive manufacturing has numerous advantages such as rapid prototyping with a fast turn-around time, low-cost entry, low waste generation and high energy efficiency. Additive manufacturing also allows fabless designing 3-D complex structures like composition and functionally graded radomes by using commercially available software and fabricating the structures at remote shared facilities for additive manufacturing, and hence lowering the costs. Besides the selection of materials and fabrication processes, the electromagnetic design of functionally graded radomes is also highly critical for the optimum radome performance for hypersonic vehicles. Radome materials and structure must be carefully designed for minimum transmission loss in the desired frequency bandwidth and minimum boresight error, in addition to the other electromagnetic, thermal, mechanical, and environmental requirements. These interrelated challenges must be addressed with a combination of materials selection, functional graded radome wall structure, electromagnetic design, and innovative manufacturing technologies.
PHASE I: Identify candidate radome materials and facilities capable of high temperature electrical, thermal, and mechanical testing of the radome materials for temperatures ranging from 500C to 1500C. Develop high temperature material characterization capabilities at small sample sizes and perform high temperature (from 500C to 1500C) material characterizations: thermal testing (thermal expansion coefficient, thermal conductivity, melting temperature, and thermal capacity), electrical testing (dielectric constant and loss tangent for different frequency bands from 50 MHz ~ 50 GHz, and resistivity), and mechanical testing (flexural strength, Young’s modulus, and Poisson’s ratio). Develop an electromagnetic design of functionally graded radome materials (FGRMs) with melting temperature >1760C, a low dielectric constant (< 5), low loss tangent (< 5~10%), high flexural strength (> 50-100 MPa), and high Young’s modulus > 50-100 GPa for hypersonic vehicles operating at temperatures >1500C. Identify a feasible and scalable additive manufacturing process for the identified radome materials. Provide experimental data along with projected performance in the target environment.
PHASE II: Develop a scalable additive manufacturing process for the FRGMs and demonstrate the production of a scale-down version of a 3-D radome with functionally graded wall structures. Perform electrical, thermal, mechanical, and reliability testing of the radome across the temperature range from 500C to 1500C (required) / >1760C (desired). Optimize functionally graded radome material design and process and conduct detailed testing of the radome materials and fabricated 3-D structures for reaching the desired electrical, thermal, and mechanical requirements stated above for hypersonic vehicles. Demonstrate a scalable manufacturing technology during production of the radome materials. Validate process repeatability and demonstrate the ability of the FGRMs to withstand the simulated aerothermodynamics heating/loading in hypersonic flight environments and to ensure reliability and structural integrity of the proposed materials. Deliver a prototype of the scale-down version of the optimized 3-D radome to US Army for evaluations. Provide complete engineering and test documentation for the development of manufacturing prototypes.
PHASE III: Expand on Phase II results by optimizing the functionally graded radome as necessary for integration into a hypersonic defense system/advanced target vehicle. Develop and execute a plan to manufacture the functionally graded hypersonic radome developed in Phase II, and assist the transitioning this technology to the appropriate missile defense prime contractor(s) for the engineering integration and testing.
1: J.B. Kouproupis, "Flight capabilities of high-speed-missile radome materials," Johns Hopkins APL Technical Digest, 13, 386 (1992).
KEYWORDS: Radome, Hypersonic Vehicle, Functional Graded, Hypersonic Aerothermodynamics Environment